For the etching and vapor deposition of substrates very often the so-called sputter technique is employed, in which in a vacuum chamber at low pressure the ions of a gas are accelerated onto electrodes, where they knock out particles, which subsequently coat or etch a substrate. Acceleration of the gas ions takes place either by means of a DC or an AC voltage, which is applied to the substrate and to the electrodes.
If an AC voltage is applied, the entire electric circuit comprises as a rule an AC voltage source, a network and a plasma impedance, i.e. of the impedance between the electrodes or between an electrode and the substrate, respectively. While the network as a rule has constant electric structural elements, which do not vary during operation, the plasma impedance can vary thereby that the fraction of ions and/or electrons varies relative to the electrically neutral particles.
If one wishes to attain a high sputtering rate during reactive medium frequency sputtering, it is necessary to operate in the transition range between metallic and fully reactive mode. Operation is to remain stable therein. However, in the case of high powers and/or cathode spacings a stable operation transition range is often difficult of attainment since an uncontrolled toggling from the metallic into the fully reactive mode occurs readily.
It is known that the impedance before the ignition of the plasma is of approximately infinite magnitude and, after the ignition, as a function of the operating point adjusts to a value of >0 Ω. It is furthermore known that the above described system composed of AC voltage source, network and plasma impedance represents a resonant circuit, whose resonance frequency varies with the plasma impedance (Society of Vacuum Coaters, Konferenzbeitrag Hüttinger Elektronik, 1999). Means, with which stable sputtering operation can be achieved, are not specified in this publication. Rather, only the state is described which adjusts under unregulated conditions with the presetting of the process parameters ‘power and gas flows’. This state defines a specific operating point at a specific plasma impedance and therewith a specific resonance frequency.
Furthermore is known a device for regulating a sputter installation in which the electric impedance of the plasma takes place by regulating the strength of the magnetic field (DE 34 25 659 A1). However, herein the feeding of the cathode takes place with DC voltage.
In another known method for coating a substrate by means of a sputter device the operating point is to be stabilized rapidly and simply (EP 0 795 890 A2). The electric power supplied to the sputtering electrode swings between two values. The power values L are selected such that at like reactive gas inflow the target of the sputtering electrode is in metallic mode during the first power value, while during the second power value, it is in oxidic mode. Regulation of the plasma impedance does not take place in this method.
Stabilization of a medium frequency sputter process without external regulation, in which the network matching is selected such that the disturbances are being counteracted, is also known (DE 195 37 212 A1=U.S. Pat. No. 5,807,470).
In addition, a method and an installation for coating at least one object with at least one layer is known, in which an ohmically conducting target is sputtered in a glow discharge operated by means of DC and AC superimposed onto it (EP 0 508 359 A1). The coating process is therein operated in unstable transition mode between metallic and reactive mode. Through the regulation this process is stabilized in unstable transition mode and specifically in transition mode in proximity to the transition into the metallic mode. As manipulated variable in the automatic control system the DC signal and/or the frequency of the DC signal and/or the frequency and/or the amplitude of the DC signal are employed. The instantaneous value acquisition is carried out through an optical method, in particular through absorption and/or fluorescence spectroscopy. Such instantaneous value acquisition requires relative high effort and expenditures.
In another known method for the reactive coating of a substrate, the frequency of the AC source during the sputter process is set such that the ions can still follow the AC field, which, at a frequency of approximately 1 kHz to 100 kHz, is the case (DE 41 06 770 A1). With the aid of a voltage effective value acquisition a tapped-off discharge voltage is supplied as DC voltage to a regulator, which, in turn, drives a magnetic valve for the supplying of a quantity of reactive gas and specifically such that the measured voltage governs the required reactive gas quantity. Thus, at constant AC frequency a voltage is measured, which, in turn, fixes the reactive gas measurement.
Furthermore a balance regulation for reactive magnetron sputtering for the optical large-area coating is known, in which the instability of the transition range between the metallic operating range and the oxidic operating range is decreased or eliminated through regulation (C. May and J. Strümpfel: Balanceregelung für reaktives Magnetronsputtern zur optischen Groβflächenbeschichtung, Vakuum in Forschung und Praxis, 2001, No. 2, pp. 79 to 84). For the instantaneous value acquisition the optical spectroscopy, partial pressure measurement and plasma impedance are drawn on herein. The regulation of the plasma impedance takes place via the discharge voltage, which is required for maintaining the power kept constant by a medium frequency current supply. However, a frequency variation of the voltage does not take place.
Lastly, a high-power generator for the medium frequency sputtering with double magnetron is also known, which ensures high power stability and matching to different load impedances (T. Rettich, P. Wiedemuth: High power generators for medium frequency sputtering applications, Journal of Non-Crystalline Solids 218, 1997, pp. 50 to 53). This oscillator comprises a resonance circuit and can provide voltages of 300 V up to more than 1200 V. Its frequency range extends from 20 to 100 kHz. The particular optimum frequency is set as a function of the material to be sputtered. Corresponding to a special layout of the free running oscillator the reaction to load fluctuations takes place within one halfwave. Load changes result immediately in frequency changes whereby a mismatch is avoided. This frequency change, however, is not utilized as a measuring value for the instantaneous value acquisition of the system impedance to be regulated.
The invention addresses the problem of providing a regulating device with which the process conditions can be kept constant.
This problem is solved according to the present invention, which relates in part to a device for the regulation of a plasma impedance, comprising a vacuum chamber, at least one electrode disposed in said vacuum chamber, said at least one electrode connected to an AC generator, wherein into the vacuum chamber a process gas can be introduced, wherein said AC generator is a free-running AC generator whose frequency adjusts to the resonance frequency of the circuitry connected to it, a reference frequency value sender, a parameter regulating device, which as a function of the difference between reference and instantaneous frequency value, regulates a parameter which affects the plasma impedance.
The invention, consequently, relates to a device for regulating a plasma impedance in a vacuum chamber, wherein at least one electrode is connected to an AC generator. This AC generator is a free running [oscillator] whose frequency adjusts to the resonance frequency of the load upon which it acts. This load consists of fixed switching elements and of a variable plasma impedance. If the plasma impedance changes, the resonance frequency also changes with it. The plasma impedance can thus be varied by acquisition of the resonance frequency and by presetting of a nominal frequency value, for example, thereby that the voltage, the current, the power or the gas inflow is varied as a function of the difference between resonance frequency and nominal frequency value.
One advantage attained with the invention comprises that the regulation suffices without a sensor, for example an optical sensor, a λ-probe or a mass spectrometer, with which the plasma state is established. A further advantage comprises that Si3N4 processes can be regulated. In addition, the operating point of the plasma process is exactly maintained.
Embodiment examples of the invention are depicted in the drawings and will be described in further detail.